BACKGROUND: Despite increasing reports of herpes simplex virus (HSV) type 1 (HSV-1)-associated anogenital herpes, there are very limited data comparing the seroepidemiological profile of and risk factors for HSV-1 and HSV type 2 (HSV-2) infection. METHODS: Sexual behaviors were examined as risk factors for prevalent and incident HSV-1 and HSV-2 infections in a community-based cohort of 1,427 HIV-negative gay men in Australia. RESULTS: The prevalence of HSV-1 and HSV-2 at baseline was 75% and 23%, respectively. The rate of prevalent infection with HSV-1, as well as the rate of prevalent infection with HSV-2, was much lower in individuals <25 years of age, and each type of infection was associated with a higher number of both male and female sex partners. The median duration of follow-up of the cohort was 2.0 years. Among participants who were susceptible to infection, the incidence rates for HSV-1 and HSV-2 infection were 5.58 and 1.45 cases per 100 person-years, respectively. In multivariate analysis, incident infection with HSV-1 was significantly associated with younger age (P=.027) and reports of frequent insertive oral sex with casual partners (hazard ratio, 3.91 [95% confidence interval, 1.23-12.44]; P=.021). Incident infection with HSV-2 was significantly associated with a variety of anal sex practices with casual partners. CONCLUSIONS: Both HSV-1 and HSV-2 were commonly sexually transmitted, and there were more HSV-1 than HSV-2 seroconversions. Public-health strategies targeted against anogenital herpes increasingly need to take into account the importance of HSV-1 infection.